The invention relates to a process for the preparation of di- and polyamines of the diphenylmethane series (MDA) by reaction of aniline with formaldehyde in the presence of acid catalyst, in which a first portion of the aniline is mixed with acid catalyst and a second portion of the aniline is mixed with formaldehyde, and these mixtures are then mixed with one another and reacted.
The preparation of MDA is generally known and is conventionally carried out by reaction of aniline with formaldehyde in the presence of acid catalysts in a continuous, semi-continuous or discontinuous process. The process is described in numerous patents and publications (see e.g. H. J. Twitchett, Chem. Soc. Rev. 3(2), 209 (1974), M. V. Moore in: Kirk-Othmer Encycl. Chem. Technol., 3rd ed., New York, 2, 338–348 (1978)).
Polyamines of the diphenylmethane series (MDA) are understood as meaning amines and mixtures of amines of the following type:
wherein n represents a natural number ≧2.
In the reaction of aniline and formaldehyde, a mixture of isomers is conventionally formed, because the CH2 group can substitute the aromatic ring of the aniline in the 2 position and/or 4 position and/or 6 position. A mixture of homologues is furthermore formed in this reaction, since n molecules of aniline can react with (n−1) molecules of formaldehyde to give a polyamine of chain length n. The composition of the isomer and homologue mixture varies according to the reaction conditions and the recipe used. In general, a high content of 4,4′-MDA (n=2) in the reaction product is desirable.
A number of methods are known for increasing the selectivity in favor of 4,4′-MDA. For example, an increase in the specific concentration of acid catalyst in the reaction mixture, an increase in the aniline excess and low reaction temperatures lead to an increase in the yield of 4,4′-MDA. Thus, e.g., DE-A-1 643 449 describes the preparation of MDA with a high content of 4,4′-MDA by reaction of aniline which has been reacted with acid beforehand with formaldehyde, the degree of protonation being at least 25%, preferably at least 50%, and even better 75–100%. DE-A-10 111 337 describes the preparation of MDA with a degree of protonation of <20%, MDA having an increased content of 2,4′ isomer nevertheless being obtained. EP-A-10 53 222 reports that a high temperature leads to MDA with an increased content of 2,4′ and 2,2′ isomers.
All of the known measures for increasing the selectivity in favor of 4,4′-MDA have the disadvantage, however, that the profitability of the process becomes lower. A higher specific amount of acid catalyst employed increases the material costs, since the catalyst is removed by neutralization during the working up and is therefore consumed. The excess amount of aniline lowers the space/time yield and increases the energy consumption during the redistillation. Lower reaction temperatures require longer reaction times in order to bring the conversion to completion.
A considerable difficulty in the industrial implementation of this process is that the chemical reactions are highly exothermic and the selectivity-determining reaction steps are very fast (H.-J. Ladwig, W. Pippel, C. Ringel, H. Oelmann, “Einfaches kinetisches Modell der Anilin-Formaldehyd-Kondensation”, Wissenschaftliche Zeitschrift der TU Dresden, 38, 1989, p. 121–126). This means that these reactions already proceed at least in part during the thorough mixing of the reactants and local temperature and concentration gradients, which reduce the selectivity, can thereby occur. For example, elevated temperatures during the mixing of formaldehyde with a reaction mixture which contains aniline and acid catalyst lead to an increase in the content of higher oligomers or ortho isomers. However, the methods known to date for suppressing selectivity losses on the basis of temperature peaks during mixing are not sufficiently effective.